Antifungal assays

Measurement of the following antifungal levels are routinely performed by the Regional Mycology Centre:

Flucytosine               Itraconazole       Posaconazole
Voriconazole             Fluconazole

Antifungal assays are required for:

• All patients receiving flucytosine.
• All patients receiving posaconazole.
• Patients receiving itraconazole (to check drug absorption and to monitor compliance).
• Fluconazole in patients on dialysis/haemofiltration.
• All patients receiving voriconazole.
• Patients failing azole therapy.
• If drug interactions are suspected.

We use bioassay methodology to measure most antifungal drugs; therefore patients on combination antifungal therapy require particular care.
Remember to specify all antifungal drugs the patient is receiving, otherwise an inaccurate result may be determined.

A review of all flucytosine levels received during 2003/4 showed that 79% had unacceptable levels of the drug, of these 27% were potentially toxic. Clearly dosage should be monitored closely by measuring levels frequently.
Itraconazole assays were performed to identify patients with absorption or metabolism problems which can lead to therapeutic failure.
In our review from the same period 54% of itraconazole levels were sub-therapeutic. All seriously ill patients receiving this drug should have levels assayed.